Low temperature chemical vapor deposited silicon dioxide thin films are routinely used as passivation and dielectric layers in integrated circuit microelectronics and in large area electronics such as displays and solar photovoltaic cells. In many of these devices, there is a need to reduce the oxide film deposition temperatures while retaining overall uniformity, homogeneity and dielectric quality. For example, in displays and solar cells, commonly employed low temperature glass substrates limit the maximum oxide processing temperature to about 600.degree. C. For dielectrics used between metals in IC manufacturing, the deposition temperature must be below 450.degree. C. to prevent the aluminum in the conductor lines from reacting with the silicon.
High quality oxides deposited at a temperature below 200.degree. C. would be highly useful to the optical coatings industry and could also help bring about the use of plastic substrates for microelectronic devices.
The deposition of high quality silicon dioxide films at temperatures below 400.degree. C. has, to date, been dominated by oxidizing plasma-enhanced chemical vapor deposition (PECVD) reactions which use tetraethyloxysilane (TEOS) or silane as the silicon precursor source. These low temperature oxides have not only been used for TFT device fabrication (see D. Buchanan et al., IEEE Electron Device Letters, V. 9, p. 576 (1988), but are also commonly employed as intermetal-dielectrics and passivation layers in the microelectronics industry (see B. Chin et al., Solid State Technology, p. 119 (April 1988).
Currently, great interest exists in reducing the deposition temperature necessary for high quality silicon dioxide film growth to allow for future applications in flat panel displays, VLSI technology and microelectronics on polymeric substrates. Even though plasma-enhanced CVD TEOS (PETEOS) and silane-based oxides can be deposited with high quality at acceptable deposition rates, these materials do have drawbacks for these envisioned applications. For example, quality PETEOS SiO.sub.2 films are difficult to achieve at temperatures below 250.degree. C. (see T. Itani et al., Mat. Res. Soc. Symp., 446, p. 255, (1997). Also, TEOS has a low vapor pressure of about 2 Torr (25.degree. C. and 1 atm.) which necessitates the heating of all delivery lines and chamber surfaces to avoid TEOS condensation. Such low vapor pressure also prevents gas metering with conventional mass flow controllers (see S. Nguyen et al., J. Electrochem. Soc., 137, p. 2209, (July 1990)).
Silane gas, conversely, is easily metered by conventional mass flow controllers (MFCs), but great care must be used because silane is a toxic and pyrophoric gas which constitutes an explosion hazard at high SiH.sub.4 concentrations.
Various other gases have been used to achieve PECVD deposition of SiO.sub.2 films. U.S. Pat. No. 5,593,741 to Ikeda discloses the use of a CVD method for the deposition of silicon oxide films through the use of TEOS and Oxygen in a plasma reaction chamber (with helium as a carrier) Ikeda's process involve alternate depositions of thin oxide layers, first without a plasma and then with a plasma which provides an ion bombardment to improve the film's properties and the film's conformance to an underlying substrate. Ikeda also indicates that tetramethylsilane (TMS) can be used in the process in lieu of TEOS.
In the examples given by Ikeda, TEOS, octamethylcyclotetrasiloxane and tris(diethylamino)silane are used as the silicon sources and the minimum substrate temperature is given as 300.degree. C. The Ikeda specification, however, quotes a range of deposition temperatures of 200.degree. C. to 400.degree. C., but provides no examples to support substrate temperatures less than 300.degree. C.
In U.S. Pat. No. 5,462,899, Ikeda describes a CVD deposition method for forming a SiO.sub.2 layer using TEOS and ozone as the principal reactants. TMS is indicated as being an alternative silicon source. The substrate temperature (for an example using triethoxyfluorosilane) is cited as 400.degree. C. U.S. Pat. No. 5,083,033 to Komano et al. describes the use of TMS as a reactant to produce a silicon oxide layer, using a focused ion beam to create an environment for the surface reaction. Guinn et al. in "Chemical Vapor Deposition of SiO.sub.2 from Ozone--Organosilane Mixtures Near Atmospheric Pressure", Materials Research Symposium Proceedings, Vol 282 (1993), indicate that TEOS is 5-10 times more reactive with ozone than is TMS, in a CVD reaction chamber, within a temperature range of 328.degree. C. to 258.degree. C.
As can be seen from the above prior art, the lowest quoted reaction temperature in a plasma chamber used for the deposition of SiO.sub.2 (where TMS is a precursor and the results are supported by experimental evidence) is about 250.degree. C.
Accordingly, it is an object of this invention to provide an improved method for the deposition of SiO.sub.2 wherein the deposition temperature is less than 200.degree. C.